A barrel engine is a type of reciprocating engine that replaces the common crankshaft with a circular plate (the swashplate). Pistons press down on a circular plate in a circular sequence, forcing it to nutate around its center. The plate, also known as a wobble plate, is typically geared to produce rotary motion.
Barrel engines are differentiated from other engines in that the cylinders are arranged in parallel around the edge of the plate, and possibly on either side of it as well, and are aligned with the output shaft rather than at 90 degrees as in crankshaft engines. This design results in a very compact, cylindrical engine, ideally suited for use in aircraft engines.